Means for controlling lubrication of hermetic compressors



July 7, 1964 1. KRAMER ETAL 3,140,041

MEANS FOR CONTROLLING LUBRICATION OF HERMETIC COMPRESSDRS Filed Jan. 9, 1961 5 Sheets-Sheet l llllh INVENTORS M4 BYW EW m TTORNEYS y 7. 1964 1. KRAMER ETAL 3,140,041

MEANS FOR CONTROLLING LUBRICATION OF HERMETIC COMPRESSORS $521421 TT0RNEY9 y 1954 1. KRAMER ETAL 3,140,041

MEANS FOR CONTROLLING LUBRICATION OF HERMETIC COMPRESSORS Filed Jan. 9, 1961 3 Sheets-Sheet 3 'u} ATTORNEYS United States Patent M 3,140,041 MEANS FDR CONTROLLING LUBRICATION 0F IERMETIC COMPRESORS Israel Kramer and Daniel E. Kramer, Trenton, N.J.,

assignors to Kramer Trenton Co., Trenton, N.J., a corporation of New Jersey Filed Ian. 9, 1961, Ser. No. 81,391 6 Claims. (Cl. 230-44) This invention relates to means for controlling lubrication of hermetic compressors, and has for an object a control of lubrication which is particularly adapted for use in connection with a plurality of hermetically sealed compressors that are operatively connected in parallel, and in which the motors are cooled either by the suction gas or otherwise, as by air stream or water jacket.

Another object is to insure against faulty lubrication of one or more of such operating compressors connected in parallel when less than all of the compressors thus connected are in operation.

Another object is to insure against the occurrence of an undesirable fall in oil level in the crankcase compartment of the operating compressor or compressors, with consequent faulty lubrication, due to increase of pressure therein caused by flow of gas from the crankcase compartment of one or more inactive compressors, when the group of compressors is, as above indicated, connected in parallel.

Another object is to provide means for attaining the above named objects which may readily be added to previously constructed and connected compressors without the necessity of performing machine work on the latter or disturbing their construction or interconnection in any manner.

A further object is to provide certain improvements in the form, construction and arrangement of the several parts of the means, and in the method of its operation, whereby the above named, and other objects inherent in the invention, may be efficiently attained.

A well known class or type of compressors such as are used, for example, in refrigeration and air conditioning systems, is the hermetically sealed compressor which is characterized by the fact that the compressor proper and its electric drive motor are enclosed in separate compartments within a single housing and the drive shaft of the motor is connected to the crank shaft of the compressor without the intermediation of shaft seals, belt and pulleys, or similarly functioning means.

When a compressor of this class or type is included in a refrigeration system, it is well known that the compressor discharge is connected by suitable conduits or pipes to condenser, receiver, expansion valve and evaporator inlet, while the outlet of the evaporator is connected by a suction conduit or pipe directly to the compressor intake manifold or to the motor compartment of the housing for the motor and compressor. It is also well understood that such a refrigeration system constitutes a closed circuit charged with a suitable refrigerant such, for instance, as Freon 12, which, in the course of its circulation through the system under the impulse of the compressor, changes in state from gaseous phase to liquid phase, and the reverse, leaving the compressor discharge as hot gas under pressure most of which is transformed to liquid phase in the condenser, thus collected under pressure in the receiver, and fed to the expansion valve, where the temperature and pressure of the refrigerant are sharply reduced so that it enters the evaporator largely as a cold liquid. In the evaporator, heat from the contents of the latter is picked up by the refrigerant with the result that it leaves the evaporator as vapor and is fed back to the compressor intake for recompression and recirculation.

3,140,041 Patented July 7, 1964 In the hermetically sealed suction-cooled type of compressor, the vapor or gas returning from the evaporator enters the compartment in which the electric motor is confined at a low enough temperature to cool the motor while, in turn, having its own temperature elevated by heat from the winding and rotor of the motor and, in this condition, flows from the motor compartment of the housing into the compressor suction manifold and through appropriate valves of the compressor cylinder for compression by the piston and discharge to the condenser as above explained. In the type of hermetic compressor which is not suction-cooled, the vapor or gas from the evaporator flows directly to the intake manifold of the compressor, and the motor is cooled by auxiliary means, such as an air stream or water jacket. The present invention is adaptable to both types.

Refrigerants of the Freon class readily dissolve in the lubricating oil such as is commonly contained in the crankcase compartment of a compressor, and this leads to foaming of the oil mixture in case of a sudden reduction in crankcase pressure, which frequently occurs upon a reactivation of the compressor following an idle period of substantial length. This foaming is often of such a violent nature as to cause entrainment of the mixture of oil and refrigerant with the vapor passing through the compressor, with the result of seriously shortening the useful life of the compressor, or even instantaneously damaging it, largely through injurious effect caused by liquid refrigerant and oil slugs upon the valve elements, pistons, pins, rods and crankshaft; while entrained oil is carried along through the condenser and into the receiver, thus lowering the level of oil in the compressor crankcase compartment to such an extent as to involve compressor break-down due to bearing failure caused by inadequate lubrication by the oil pump with intake commonly located at the bottom of the crankcase compartment for feeding oil through ducts to the working parts of the compressor.

In order to guard against the injurious effect just described, a practice has developed of isolating the crankcase compartment of the compressor from the main suction gas stream and, in suction-cooled hermetics from the motor compartment by means of a partition through which the motor shaft generally passes. The crankcase compartment is sized to enable it to hold the major portion of the oil in the system. To make possible the return of oil to the crankcase compartment from the suction gas stream a discriminating check valve is installed iri the wall of the compressor between the refrigerant vapor inlet of the compressor and the crankcase compartment, which valve serves not only the dual function of draining oif particles of oil that are entrained with the refrigerant vapor and causing the same to drop back into the crankcase compartment, while enabling the venting from the latter of refrigerant vapor that enters by way of passing between the piston and cylinder upon compression strokes, which is known as blow-by vapor; but also the function of closing and preventing foaming oil from leaving the crankcase compartment. Such a discriminating check valve is well known in this industry as to its construction, mode of operation, and its use for the purpose just described, so that no detailed description is regarded as necessary. In hermetically sealed, suction-cooled compressors the difficulty just mentioned is aggravated due to the fact that the vapor returning from the evaporator to the compressor through the suction conduit enters the motor compartment of the motor-compressor housing before reaching the compressor, and oil entrained with the refrigerant vapor drips into the said motor compartment. Thus, the latter, in effect, becomes an oil separator and needs to be provided with means for passage of the oil gathered therein to the compressor crankcase compartment. This need has been served by 40 F. in the evaporator.

a incorporating a check valve positioned near the bottom of the partition that separates the motor compartment from the compressor crankcase compartment, which valve permits flow to the latter but inhibits reverse flow and thus equalizes the oil level in the motor compartment and crankcase compartment unless the vapor pressure in the latter rises above that in the former.

When hermetic compressors are used in low temperature refrigeration systems, for quick freezing of substances for example, and what is known in the industry as high compression ratio develops, there is a substantial increase in the portion of the compressed vapor which escapes as blow-by vapor downwardly between piston and cylinder into the crankcase compartment during compression strokes of the piston. This can, and frequently does, increase the crankcase pressure in the suction-cooled type of compressor to such a degree as to prevent passage of accumulated oil from the motor compartment to the crankcase compartment with ensuing and dangerous depletion of oil in the latter. To cope with this problem, reliance has been placed upon the discriminating check valve hereinabove described to which has been added an uprising tube known in this trade as a snorkel that projects into the suction manifold of the compressor and becomes a part of the crankcase compartment insofar as the presence of oil and the circulation of refrigerant gas to avoid the existence of gas pockets is concerned. Thus there is no check valve near the base of the partition as above described in connection with the suction-cooled type of compressor, but the oil precipitates from the stream of refrigerant gas in the compressor manifold and through the discriminating check valve into the crankcase compartment, from which it can flow into the motor compartment through the above mentioned openings in the partition. In this non-suction-cooled type of compressor the above mentioned snorkel is also omitted.

Proper oil level in the compressor crankcase compartment of both types of compressors is a matter of such vital importance that it is common practice for the manufacturer to install a sight glass in the wall of the crankcase compartment at the desired level for visual observation by the operator in charge.

Occasion frequently arises when, for certain reasons such, for instance, as etficiency and economy, it is desirable to link in parallel a plurality of these hermetically sealed compressors of either type, which naturally presents the problem of equalization of oil level in the several crankcase compartments, and its maintennace. This condition has been met by interconnecting the crankcase compartments, at sight glass level, with a pipe or conduit usually of the same diameter as the sight glass, the lower portion of which pipe provides for the travel of oil to equalize level, while the upper portion serves as a vapor duct for equalizing pressure. While this arrangement has worked with satisfaction when all the linked compressors are operating, trouble has been encountered when, intentionally or otherwise, less than all the compressors are in action, especially when the refrigeration system is developing low freezing temperatures, eg minus to minus The apparent cause of this trouble is the flow of vapor through the interconnecting conduit from the crankcase compartment of the idle compressor or compressors to the active one or ones in an amount which raises to such an excessive degree the pressure in the crankcase compartment of the active comabove described cannot exhaust the vapor with sutficient speed. to permit equalization of oil level, and injurious depletion of lubricating oil in the crankcase compartment occurs. In the suction-cooled type, this super or excessive pressure in the crankcase compartment of a working compressor or compressors can prevent needed flow thereinto of oil that has collected in the motor compartment of the motor-compressor housing; and this gives rise to a further ditliculty because the high level of oil in the motor compartment leads to undesirably high compressor discharge temperature with its concomitant disadvantages.

The present invention proposes to solve the problem by providing for connecting the parallel-linked hermetic compressors in such manner as to permit interflow of oil between their crankcase compartments for equalization of oil level, while so reducing the flow of vapor from nonoperating to operating compressor or compressors as to avoid excessive rise of pressure in the latter with consequent oil depletion and loss of efficiency with likelihood of damage.

In brief summary, the invention comprises the interconnection of the crankcase compartments of all the parallel-linked hermetic compressors with an oil level equalizer conduit, tube or pipe the internal passage of which is of such restricted size as largely to reduce or limit the flow of refrigerant vapor therethrough while permitting the adequate passage of oil. This may suitably be accomplished by the use of a tube or pipe of small bore, or by using one of larger internal diameter fitted with small diameter nozzles, or with a plug, or the like, having a suitably calibrated orifice, the calibration being such as to respond to a predetermined pressure. This conduit, tube or pipe may also be of U form, or similar shape, to provide any desired head height and thus constitute a trap for allowing flow of oil while impeding the flow of vapor therethrough, and also to permit increase in length for the same functional effect. Preferably, the points of interconnection of the crankcase compartments are at the hereinabove mentioned oil level sight glasses that are commonly carried in screw plugs threaded into the sides of the crankcases, or in flanged plugs screw bolted in place. The connection of the conduit, tube or pipe of this invention may conveniently be accomplished by the provision of adapters, each of which has the general form of a hollow fitting that is externally threaded or shaped at one end for entering the bore which receives the oil sight glass, and is internally threaded or shaped at the opposite end for seating of the sight glass plug, which latter is removed for installation of the adapter. Each extremity of the interconnecting conduit, tube or pipe passes through an opening in the lower portion of an adapter, the union of the parts at this point being readily made by use of a fitting such, for instance, as a tubular coupling threaded into the adapter opening and united to the interconnecting tube or pipe in any satisfactory manner, as, for instance, by compression or soldering. The inner end of the tubular coupling should preferably extend within the adapter to a distance equal to the desired oil level. In the manufacture of some compressors, the casting is machined to provide an outlet separate and apart from the oil sight 22 on its Way to the cylinder 13.

perspective by the oil level equalizer conduit, the adapter being of the screw threaded form;

FIG. 3 represents on a slightly smaller scale and in detail vertical section, the flanged screw bolted form of adapter with a portion of the oil equalizer conduit connected thereto, affixed to a crankcase compartment;

FIG. 4 represents a view taken at right angles to FIG. 3, and in full elevation; and

FIG. 5 represents a view similar to FIG. 1, but illustrating compressors of the non-suction-cooled type.

Referring specifically to the form of the invention exhibited in FIGS. 1 and 2, the housing of the first hermetic compressor is denoted generally by 1, and that of the second compressor by 2. It will be observed that the said housings are separated into two compartments marked 3, 4, and 5, 6, by respective partitions 7 and 8. In compartment 3 of the first compressor is mounted the electric drive motor, indicated generally by 9, while the motor for the second compressor, only partially shown and marked 10, is similarly mounted in compartment 5 of housing 2. Compartments 4 and 6 constitute, at their lower portions, the crankcases of the compressors.

The compressor proper of the first unit, which may be multi-cylinder, is shown in some detail, it being regarded as unnecessary to illustrate this in duplicate in the second housing. Referring to the former, its crank shaft 11 is directly connected to the motor 9, and one of its pistons is marked 12 which reciprocates in cylinder 13 cooperating with the usual intake valve 14 and dis charge valve 15 for the flow of hot gas or vapor through discharge conduit 16, which latter is connected at 17 with the discharge of the second unit, to the condenser, not shown. The suction conduit from the outlet of the evaporator, not shown, bears the reference numeral 18, and its branches 19 and 20 extend to the motor compartments 3 and 5, of the two units, the usual inlet valves being omitted. Compartment 3 is formed with a port 21 that permits the suction vapor passing through the motor compartment to enter compressor intake manifold It will be understood that the second compressor unit is similarly constructed. From the foregoing, it will be evident that these compressors, thus linked in parallel, have a single or common discharge line or conduit and also a single or common suction line or conduit.

A check valve 23 near the bottom of partition 7 permits oil that has precipitated from the passing refrigerant vapor to flow from motor compartment 3 to the compressor crankcase compartment 4; while a discriminating check valve 24 is located in the upper part of compartment 4 to open communication between said compartment and the inlet manifold 22. This valve is adjusted to permit the venting of blow-by vapor from the crankcase compartment into the suction manifold; but to close when the hereinabove explained foaming of the mixture of oil and refrigerant occurs due to sudden reduction of crankcase pressure, and prevent foaming oil from leaving the crankcase compartment. As above indicated, such a valve is well known and available in the open market, requiring no further showing. To increase the venting effect of this valve, it is preferably fitted with an uprising tubular extension or snorke 25.

Coming now to the particular subject matter of the present invention, it will be noted that the crankcase portion of each housing compartment 4 and 6 is provided with a bore or hole 26, 27 (FIG. 2) in its side wall which is threaded for the seating therein of the customary oil level sight glass that is commonly set in a screw plug. Into each of these bores or holes we fit a hollow adapter which constitutes an important element of our lubricating controlling means and is denoted generally by 28. As the two adapters are alike, only one will be described in detail, corresponding reference numerals being applied to the other. One end 29 of the adapter is exteriorly threaded to mate with the interior threading of the bore or hole in the crankcase, while its other end 30 is recessed and interiorly threaded to receive the oil level sight glass plug 31 with its glass or window 32.

The two adapters are connected by an oil level equalizer conduit, tube or pipe 33 that is preferably of Wide U shape, and the internal passage through which is so constricted as substantially to reduce or limit the flow therethrough of vapor while permitting the adequate flow of oil.

The extremities of this equalizer conduit are secured through the undersides of each adapter to communicate with the hollow interior thereof and, as both securing means are the same, only one will be explained with similar reference numerals assigned to the other. It will be seen by reference to FIG. 2, that the ends of conduit 33 are flared, as indicated at 34, 34, and gripped between one threaded end of the male member 35 of a half union coupling and its cooperating female member 36. The other end of the coupling is threaded into an opening 37 in the adapter 28, and the coupling is also formed with a suitably shaped tool flange 38. A nozzle 39 is either fixed in any suitable manner to, or formed integrally with, the male member 35 of the coupling, which nozzle extends into the hollow adapter 28 for a distance such that it is approximately at the desired oil level in the compressor crankcase, and the internal size of the nozzle provides the restricted passage, above mentioned, through the conduit 33 which substantially reduces or limits the flow of vapor while permitting sufficient flow of oil. We have found that an internal nozzle diameter of about three tenths of an inch is generally satisfactory. As an alternative, the Whole conduit 33 could have such a minute internal diameter or, as another alternative, one or more discs or plugs with such a small opening therethrough could be fitted within the conduit. It will be understood that the interchange of only a small quantity of oil is needed for the purpose of maintaining satisfactory equalization of oil level in the several crankcases involved and that a conduit having the restricted passage described is adequate for this function while, at the same time, such restriction prevents the passage through the conduit of vapor in amount sufficient to overtax the discriminating venting valve with which the crankcase compartments are fitted and thereby cause failure of oil flow to the crankcase compartment of the active or operating compressor or compressors.

All the parts above mentioned may be formed of any suitable material, the choice of which will be well within the knowledge of those acquainted with this industrial field, but copper may be suggested for the conduit 33, and a ferrous metal for the other elements.

The oil level is appropriately indicated in the several views of the drawings, but no reference numeral has been applied thereto.

In adaptation, when a plurality of the above described hermetic compressors are linked in parallel, the lubrication controlling means or device which constitutes the subject matter of this invention may very readily be embodied in the assembly by removing from the crankcases of the compressors the oil level sight glass plugs 31 and inserting the same in the threaded recesses formed therefor in the adapters 28. With the bores or holes 26 in the walls of the crankcases previously occupied by the oil level sight glass plugs thus vacated, the adapters 28 are threaded thereinto. Following this, the extremities of the oil level equalizer conduit 33 are affixed to the adapters by the half couplings 35, which latter are screwed into the adapters to such a distance that the ends of the nozzles 39 are substantially at the desired oil level in the crankcases. It may be emphasized that this incorporation of the subject matter of our invention into the assembly of the hermetic compressors linked in parallel entails only a very simple operation which is entirely devoid of any machining, or the like, of the parts, especially the crankcases; as it is known that the possibility of fouling the lubricating system with iron chips incident to machining is a very dangerous aspect which it is of importance to eliminate.

With the parts thus united, the operation of the invention in maintaining the proper oil level in the crankcase compartment of the active compressor or compressors of the plurality linked together will be as described in a foregoing part of this specification, the conduit 33 serving to permit interflow of an adequate amount of oil from one crankcase compartment to another, in either direction as the occasion may require, while preventing the flow of an excessive amount of vapor from the crankcase compartment of one or more inactive compressors to the crankcase compartment of one or more active compressors with the consequent increase of pressure in the latter that can obstruct the flow of lubricating oil from the motor compartments thereof as hereinabove explained.

The modified form of the invention, represented in FIGS. 3 and 4, differs from the form of FIGS. 1 and 2 in that the bore or hole in the compressor crankcase, here marked 40, for reception of the oil level sight plug is not threaded, and in that the adapter, here indicated by 41, has a sliding fit in the said bore or hole, being secured in place by screw bolts or cap screws 42. All the other parts are the same in both forms of the invention and bear the same reference numerals. A suitable gasket is shown interposed between the adapter 41 and the crankcase, and it should be said that gaskets may be inserted wherever deemed necessary or desirable between any meeting parts in either form of construction.

Adverting now to the form of the invention represented in FIG. 5, which shows the type of hermetic com pressor that is not suction-gas-cooled, but in which the motor is cooled, as by an air stream or water jacket (not shown), the structure is very similar to that of FIG. 1 and many of the parts bear the same reference numerals. The important difference between the embodiment of FIG. 1 and that of FIG. 5 resides in the fact that the suction conduit, here marked 43, is connected through its branches 44, 45, with the suction or intake manifolds of the compressors, indicated by 46, 47 in this figure, instead of leading to the motor compartments 3, 5, of the housings 1, 2. Thus, in this arrangement, there is no check valve in the partition denoted by 48 that separates the motor compartment from the crankcase compartment, the said partition having a series of holes or apertures formed therethrough, two of which are shown and indicated by 49, 50, both above and below the crank shaft of the compressor, for the free circulation movement of oil and vapor between the motor compartment and crankcase compartment. And, consistent with the fact that the suction conduit is connected directly to the intake manifolds of the compressors, there is no port, similar to the port 21 of FIG. 1, that provides communication between the motor compartment and the manifold. Again, the discriminating check valve 24 is not equipped with a snorkel such as the element 25 illustrated in FIG. 1.

The remainder of the construction depicted in FIG. 5 is like that of the showing set forth in FIG. 1, and the operation is the same with the exception that there is no flow of suction gas through the motor compartment to the intake manifold of the compressor, and there is no check valve control of the passage of oil from the motor compartment to the crankcase compartment, the accumulation of oil in the motor compartment not being due to precipitation from the suction gas stream in the motor compartment but due to the precipitation of oil from the suction gas stream into the intake manifold,

through the discriminating check valve into the crankcase compartment, and from the latter through the perforations in partition 48 into the motor compartment.

In view of the fact that the oil level equalizing means constituting the novel concept of our invention is combined with the form of the invention exhibited in FIG. 5 in the same manner as with the form exhibited in FIG. 1, and follows the same mode of operation in both cases, it is not deemed necessary to enlarge in further detail c upon the representation of FIG. 5; but it is felt appropriate to note that the two forms of adapters illustrated in FIG. 2 and in FIGS. 3, 4, are equally applicable to the types of compressors exhibited in FIGS. 1 and 5, and that the manner in which the parts function is the same in both cases.

The structure of the invention is not alone regarded as novel, as the adapters per so are likewise considered to have that characteristic.

It will be understood that various changes may be resorted to in the form, construction, arrangement and material of the several parts without departing from the spirit or scope of the invention; and, hence, we do not intend to be limited to details herein shown or described except as the same may be included in the claims or be required by disclosures of the prior art.

What we claim is:

1. In a refrigeration system, or the like, including a plurality of hermetically sealed suction cooled compressors linked in parallel and each having a crankcase compartment and a motor compartment with means for allowing drainage of oil from the latter to the former and positive vapor venting means in each crankcase compartment adequate to maintain a vapor pressure therein at least as low as the pressure in the associated motor compartment, means for maintaining a desired level of lubricating oil in the crankcase compartments of all operating compressors when less than all are in operation, said means comprising an oil level equalizer conduit adapted to be externally attached connecting the crankcase cornpartments of all the compressors at the desired oil level, the internal oil flow passage of said conduit being dimensioned and adapted to limit the flow of vapor therethrough from the crank case of any non-operating compressor to the crank case of an operating compressor and prevent rise of vapor pressure in the latter beyond the capacity of its venting means to maintain therein a pressure at least as low as the suction pressure in the associated motor compartment for facilitating oil drainage from the latter to the former.

2. A system as defined in claim 1, in which the hermetically sealed compressors have a common discharge conduit and a common suction conduit.

3. A lubrication controlling device designed, constructed and adapted specifically for incorporation in a refrigeration system, or the like, including a plurality of hermetically sealed compressors linked in parallel and each having a crank case compartment and a motor compartment and an oil level sight glass opening in the wall of its crankcase compartment, said device comprising, a hollow adapter designed and suited operatively to seat with a mating fit in each oil level sight glass opening, and an interconnecting oil level equalizer and vapor flow control conduit having its extremities united with the adapters, the flow passage through the conduit and adapters being so restricted in size as to limit the flow of vapor and prevent undesired rise of pressure in the crankcase compartment of any operating compressor while allowing adequate flow of oil thereto.

4. A device as defined in claim 3, in which each adapter has an aperture in its side wall formed operatively to receive an extremity of the conduit.

5. A device as defined in claim 3, in which each adapter has one end formed operatively to seat in an oil level sight glass opening and the other end formed to receive an oil level sight glass in operative position.

6. A device as defined in claim 5, in which each adapter has its inner end exteriorly threaded for operative seating in an interiorly threaded oil level sight glass opening and its outer end interiorly threaded, and which also includes a plug embodying an oil level sight glass, the said plug being exteriorly threaded for seating in the interiorly threaded outer end of the adapter.

flkeferences on following page) 9 References Cited in the file of this patent UNITED STATES PATENTS Hulf Apr. 30, 1912 Johnson Apr. 18, 1939 5 Jordan Aug. 26, 1941 Lamberton Mar. 14, 1950 Penn Oct. 4, 1955 10 Neubauer Jan. 20, 1959 Soumerai Mar. 1, 1960 Nicholas et a1. Aug. 9, 1960 Hamilton Oct. 18, 1960 Heitchue Dec. 6, 1960 Tower Dec. 6, 1960 Berger et a1. May 8, 1962 Soumerai Aug. 28, 1962 

1. IN A REFRIGERATION SYSTEM, OR THE LIKE, INCLUDING A PLURALITY OF HERMETICALLY SEALED SUCTION COOLED COMPRESSORS LINKED IN PARALLEL AND EACH HAVING A CRANKCASE COMPARTMENT AND A MOTOR COMPARTMENT WITH MEANS FOR ALLOWING DRAINAGE OF OIL FROM THE LATTER TO THE FORMER AND POSITIVE VAPOR VENTING MEANS IN EACH CRANKCASE COMPARTMENT ADEQUATE TO MAINTAIN A VAPOR PRESSURE THEREIN AT LEAST AS LOW AS THE PRESSURE IN THE ASSOCIATED MOTOR COMPARTMENT, MEANS FOR MAINTAINING A DESIRED LEVEL OF LUBRICATING OIL IN THE CRANKCASE COMPARTMENTS OF ALL OPERATING COMPRESSORS WHEN LESS THAN ALL ARE IN OPERATION, SAID MEANS COMPRISING AN OIL LEVEL EQUALIZER CONDUIT ADAPTED TO BE EXTERNALLY ATTACHED CONNECTING THE CRANKCASE COMPARTMENTS OF ALL THE COMPRESSORS AT THE DESIRED OIL LEVEL, THE INTERNAL OIL FLOW PASSAGE OF SAID CONDUIT BEING DIMENSIONED AND ADAPTED TO LIMIT THE FLOW OF VAPOR THERETHROUGH FROM THE CRANK CASE OF ANY NON-OPERATING COMPRESSOR TO THE CRANK CASE OF AN OPERATING COMPRESSOR AND PREVENT RISE OF VAPOR PRESSURE IN THE LATTER BEYOND THE CAPACITY OF ITS VENTING MEANS TO MAINTAIN THEREIN A PRESSURE AT LEAST AS LOW AS THE SUCTION PRESSURE IN THE ASSOCIATED MOTOR COMPARTMENT FOR FACILITATING OIL DRAINAGE FROM THE LATTER TO THE FORMER. 